Surface acoustic wave (SAW) devices used as sensors in measurement systems are known. For example, a tire pressure monitoring system (TPMS) helps to improve fuel economy and improve handling and safety by warning the driver about low tire pressure. TPMS is a vehicle-embedded system detecting the tire pressure by analyzing the difference between the wheel speeds or by direct measurement of pressure and temperature. Systems like a direct TPMS system typically consists of one central transceiver in the vehicle and four sensors mounted on the wheel rim, or valve stem to sense pressure and temperature inside the tire, and to organize data transmission to/from the central transceiver.
Various other SAW sensor applications are known in the art. In particular, many different techniques have been proposed for sensing the temperature of a component in an industrial process or system. SAW based pressure and temperature sensors, can be used in industrial and commercial systems to convey pressure and temperature values during processing operations such as filling, pumping, drilling, evacuating, dispensing, sealing, machine control, and condition monitoring applications in automotive, food and beverage, dairy, petroleum, medical, aircraft and surface transportation applications.
The majority of prior art sensors are direct active systems, some utilizing a silicon micro-electro-mechanical system (MEMS) capacitive or piezo-resistive based sensor powered by a battery. Where several sensors are utilized throughout a target system, pressure and temperature information is transmitted by radio frequencies from each sensor location (e.g., each of the wheels on a motor vehicle) to a central transceiver, located in or around the electronic control unit (ECU) and displayed as either a number or a warning indicator. The problem associated with using such prior art systems in, for example, a TPMS environment is that the need to remove the tire for access to the batteries, and the need to rebalance the tires after battery replacement, together with the disposal of worn out batteries are the major shortcomings of direct sensing systems. Batteries inside tires add weight, have limited life and cannot be replaced. Furthermore, they have inherent battery related performance issues such as temperature dependent voltage and current variations of the battery. These type of variations can cause inaccuracy in the sensor output that result in pressure or temperature reading errors.
Conventional wireless systems are not durable and are expensive to design and produce. The sensors and transmitters must also be able to withstand the harsh environment, such as when used inside a vehicle tire that includes high and low temperatures, shock and vibration, and centrifugal forces from tire rotation. Although they also feature wireless communication of the pressure and temperature values to a remotely placed central transceiver, they are difficult to install and service, and have significantly more electronics along with a battery in the wheel sensor to enable communications. More electronics in the wheel sensor and the previously mentioned battery voltage and current errors tend to make these types of devices less reliable. Also the wireless sensors utilizing a battery are not suitable for applications requiring intrinsically safe operations such as for e.g. petrochemical industry.
A need therefore exists for an improved wireless and batteryless sensor apparatus and easy to install packaging system, which can be integrated into a moving or rotating object such as for e.g. tire, industrial apparatus etc. and interrogated wirelessly, and that the sensors are ultimately more efficient and more reliable with fewer components than presently implemented sensors. Such an apparatus is described in greater detail herein.